Wireless devices, such as Smartphones, are more and more replacing various other daily items individual persons carry along with them ranging from credit cards to home keys. Typically a home door is opened or an item is bought from a shop by swiping a mobile device, such as a phone, next to a receiver and optionally inserting a PIN code or a password on the phone. If the Smartphone is stolen or lost somewhere, anyone finding it could make payments or open doors on behalf of the rightful owner. Instead of having merely the possession of the Smartphone some form of extra security is required for authenticating the correct user. Usually a simple solution is to use a PIN code or a password on the device every time it is used but this is rather tedious since it requires user interaction and additionally remembering passwords or PIN codes, respectively. Further, a bystander may see the password or recognize the PIN code and if he or she steals the phone, he or she could have access to everything that is authenticated with the phone and the password.